Provided is a method of manufacturing a semiconductor device. According to the method, a first buried oxide layer is formed in the semiconductor substrate in a first region, such that a first semiconductor layer is defined on the first buried oxide layer. An active portion is defined by forming a trench in the semiconductor substrate in a second region. A capping semiconductor pattern is formed on a top surface and an upper portion of a sidewall of the active portion. An oxide layer is formed by oxidizing the capping semiconductor pattern and an exposed lower portion of the sidewall of the active portion, such that the oxide layer surrounds a non-oxidized portion of the active portion. The non-oxidized portion of the active portion is a core and one end of the core is connected to a first optical device formed at the first semiconductor.